From the combination of imaging and spectroscopic observations at ultraviolet (UV), optical, and (far-) infrared (IR) wavelengths, a reasonably robust picture of how galaxies evolve across the past 9–10 Gyrs of cosmic time (corresponding to redshifts \(z < 3\) ) has been established. Similar to the local universe, galaxies exhibit a tight relation between star-formation rate (SFR) and stellar mass (the star-forming main-sequence; [1]}, [2]}) as found by robust total SFR measurements from far-IR observations. However, galaxies in the earlier universe are significantly more star-forming on average and the fraction of quiescent galaxies is decreasing strongly at redshifts higher than \(\sim \) 2 [3]}, [4]}, [5]}. This is also the epoch when the cosmic SFR density reaches the peak over cosmic time [5]}. Along with changing SFRs, the environment in which stars are being formed evolves across cosmic time. Galaxies become more dust- and metal-enriched over time as their stellar masses are being built up by galaxy–galaxy mergers and the inflow of new gas to fuel star formation [7]}, [8]}. The rate of galaxy–galaxy major mergers (with a mass ratio smaller than 1:4) as well as the gas fractions of galaxies are observed to increase towards earlier cosmic times [9]}. The overall increase in SFR as a function of redshift is found to be coupled with increasingly large gas reservoirs, while the vertical position on the main-sequence at a given stellar mass and redshift is dominated by changes in star-formation efficiency [10]}, [11]}. Finally, high-resolution space-based observations of the light distribution of galaxies at early times show that they are significantly more irregular and dominated by clumps of star-forming gas and minor mergers [12]}, [13]}, [14]}. Spectroscopic observations of the H\(\alpha \)  emission line of large samples of galaxies at \(z\)\(\sim \) 2 have confirmed this picture by revealing a significant amount of dispersion dominated and merging galaxies (e.g., [15]}).
